Magnesium deficiency in cannabis plants – how to spot it, and what to do about it
After the three main nutrients (nitrogen, potassium and phosphorous), magnesium is the most crucial secondary nutrient for healthy growth and development of cannabis plants. Without it, plants wither, fail to thrive, and eventually die. What does a cannabis plant with magnesium deficiency look like, and what can be done to help it recover? Help is at hand!
Why do cannabis plants need magnesium?
The reason that plants – and not just cannabis plants – need magnesium is simple: it is essential for photosynthesis. Without it, leaves cannot absorb and process light into energy. Plants with magnesium deficiency will eventually starve to death, even if they are getting the correct amount of hours of light.
How can magnesium deficiency in cannabis plants be identified?
As the primary ‘building block’ of chlorophyll, magnesium is what gives it, and therefore the plant itself, a healthy bright green colour. The first and most easy to identify sign of magnesium deficiency in cannabis plants is the leaves showing a worrying fade of green to pale green to yellow.
This is known as chlorosis. Although there are other problems that also cause chlorosis, magnesium deficiency shows up first around the edges and in between the veins on the leaves, rather than all over them or from the stems outwards or tips inwards.
The first leaves to show these signs will be the oldest ones, and the ones closest to the bottom of the plant (often, these are the same ones). This is caused by the plant withdrawing magnesium from these leaves in order to send it to the newer ones, in an effort to keep itself alive (in much the same way as the human body pulls blood away from the extremities in cold weather to keep the vital organs warm).
The leaves might also start to feel crunchy and dry, although it is not advisable to wait until this happens before starting treatment!
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What are the dangers of magnesium deficiency in cannabis plants?
Not only is magnesium essential for photosynthesis, it is needed for the absorption of other vital nutrients. These especially include nitrogen and phosphorus, two of the three primary nutrients. As well as being unable to photosynthesise, plants will be unable to develop sufficiently large and healthy root systems and flowers. This is obviously bad news for the plants themselves, and for anyone hoping to harvest those flowers!
Indeed, cannabis plants with magnesium deficiency may not even make it to the flowering stage. As its supply of magnesium is depleted, more and more of the leaves will turn yellow, then brown, and eventually drop off, causing the plant to die.
What causes magnesium deficiency in cannabis plants?
Unsurprisingly, the first culprit is not enough magnesium in the soil. It is an unfortunate fact that magnesium can be quite easily washed away by flushing or overwatering. The good news is that depending on where you live, your tap water may actually contain useful levels of magnesium!
Even if there are sufficient levels of magnesium in the soil, the plant may not be able to make use of them. A substrate with a pH value that is too low, i.e. very acidic, prevents the roots from absorbing magnesium even if it is plentiful. It is also important to know that adding more magnesium to a substrate that already has enough can make the situation worse.
Other necessary nutrients can be ‘locked out’ by an excess of magnesium in the substrate, and suddenly the gardener has multiple nutrient deficiencies to deal with, not just one!
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How can magnesium deficiency in cannabis plants be corrected?
Testing levels of magnesium in a substrate is difficult without laboratory analysis, something that very few people have easy access to. The best solution would be to correct everything that could be causing problems rather than attempting to tweak one thing at a time.
The most common way to correct pH and remove excess nutrient build-up in a non-soil substrate such as coco coir, or in a hydroponic system, is a thorough flush with pH-balanced water followed by another flush with pH-balanced water supplemented with regular cannabis nutrients. In hydro systems, the pH should be 6.0 – 6.5 for the best magnesium uptake.
The exact type of nutrient ratio will depend on whether the plants are in the vegetation or flowering phase. Obviously, it’s important to make sure that the nutrient solution used contains magnesium. It is also recommended to check that it contains calcium, as calcium deficiency often occurs in tandem with magnesium deficiency.
Growing cannabis plants in soil, especially in open ground rather than pots, makes them far less likely to suffer from magnesium deficiency. However, it does not guarantee immunity! In soil, the pH should be 6.0 – 7.0. The same flushing technique can be applied as above, and then a specialised magnesium and calcium supplement (available at good garden centres) can be added.
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Quick fix for magnesium deficiency in cannabis plants
As a quick-fix emergency measure, mixing magnesium sulphate with water to make a foliar spray is often effective. Magnesium sulphate is sold under the name Epsom salts, and is easy to get online or from large pharmacies.
A magnesium foliar spray is made by mixing one tablespoon of Epsom salts with four litres of water. This mixture is then sprayed onto the plants every three days, with a clean water spray every ten days to ensure residue does not build up on the leaves.
We hope that this article has been informative about magnesium deficiency in cannabis plants! Do you have any tips for us? Please leave them in the comments below. Do you have questions about magnesium, or other types, of deficiencies? Please consult our forum, where a thriving community can help you find the answers.
Magnesium deficiency is a common problem for gardeners, and cannabis plants are not immune to it! Luckily it’s quite easy to detect and fix. This article explores what causes magnesium deficiency, what it looks like, and what can be done to reverse it.
Problem: A light green or yellow coloring will begin to show on the veins and edges of the lower & older leaves – this is one of the classic signs of cannabis magnesium deficiency. You may also see red stems.
Magnesium is a mobile nutrient, which means that the plant can move it from old leaves to new leaves.
If you don’t react to it promptly, a cannabis magnesium deficiency can spiral out of control and cause your plant to lose a lot of lower leaves quickly. The plant will pull magnesium out of older leaves and bring them to the newer leaves. That’s why a magnesium deficiency usually appears towards the bottom of the plant and on older, less important leaves.
The edges of the leaves may become yellow or bright green and may start feeling crispy to the touch. This crispiness around the edges is different from nutrient burn, which does not lighten the margins inside the leaves.
You may see red stems with a magnesium deficiency, though not always.
Sometimes you will also get light brown spotting within the margins or along the edges if the problem continues to get worse, though this may be partially other deficiencies, which often happen alongside a magnesium deficiency.
Magnesium deficiencies are easy to prevent and fix once you know what to do. Read on below to learn how.
Solution For Magnesium Deficiency in Cannabis
A magnesium deficiency sometimes happens after supplementing plants with something that contains calcium but not magnesium, such as agricultural lime (non-dolomite lime), egg shells, etc. This won’t happen with proper cannabis nutrients. If you’re using good nutrients, cannabis plant may still show signs of a magnesium deficiency if the pH at the roots is too low, especially in hydro. That is because when the pH of your root zone is not in the correct range, your cannabis cannot properly absorb magnesium through the roots.
Often with this deficiency, the magnesium is present, but the roots cannot absorb the magnesium properly due to an improper pH. Therefore it is very important to maintain the correct pH (and make sure the pH does not get too low / acidic) in order to avoid a magnesium deficiency.
Growers using Coco Coir or Reverse Osmosis (RO) water usually need to supplement their plants with extra Calcium & Magnesium in addition to regular nutrients. Treating coco coir with Cal-Mag and supplying extra throughout your grow is recommended for growers in coco coir, or those using RO water.
Adding more magnesium to a system when there is a pH lock-out will probably not help because the plant will not be able to absorb any magnesium until the pH has been corrected. If there’s already enough magnesium, adding more can cause other apparent deficiencies by locking out other nutrients from the plant.
Please note: Once a magneisum deficiency is cleared up, the problem (yellowing lower leaves) will stop spreading to other older leaves, usually within a few days. Please note that leaves which have been damaged by a magnesium deficiency will probably not recover or turn green, so you want to pay attention to other growth for signs of recovery.
- In soil, magnesium is best absorbed by the roots in the 6.0 – 7.0 pH range (some growers say a 6.5 – 7.0 pH is best if you suspect a magnesium deficiency)
- In hydro, magnesium is best absorbed by the roots in the 6.0 – 6.5 pH range (in hydro, it’s generally recommended to keep the pH between 5.5 – 6.5, but magnesium specifically tends to be best absorbed above 6.0)
If you suspect your growing cannabis plant has a magnesium deficiency, flush your system with clean, pH’d water that contains a regular dose of cannabis-friendly nutrients that includes magnesium. This will remove any nutrient salts that may be affected the uptake of magnesium and help restore pH to the proper levels..
To supplement with extra Magnesium…
Calcium and magnesium deficiencies often appear together in cannabis. Many growers decide to purchase some sort of Calcium-Magnesium (often called Cal-Mag) supplement for their grow room in case one of these common deficiencies appear.
Listed below are common cannabis Calcium supplements, along with some general information about each one. After supplementing with Cal-Mag and correcting the pH, you should expect to see new healthy growth within a week. Remember, the old leaves will probably not recover, but new growth should be green and healthy.
Cal-Mag is Well Suited For Hydro, Coco Coir, or Soil
General Hydroponics CaliMagic is a calcium and magnesium plant nutrient supplement. General application is to mix 1 tsp (5ml) of CaliMagic into each gallon of water. I have used Calimagic several times with great results.
Organic Dolomite Lime – For Soil Growers
If you’re looking for a way to supplement magnesium in your organic or soil setup, I highly recommend a product called “Dolomite Lime.”
Note: The finer the dolomite, the more quickly it will be available to nutrients
Dolomite is a good source of calcium and magnesium and can be mixed with your soil. The great thing about dolomite is it works slowly over the course of a few months.
Dolomite has a neutral pH of about 7.0 and will help buffer pH in soil so it’s easier to maintain the correct neutral pH range which is optimum for cannabis growth, especially in acidic soils.
You can buy Dolomite Lime online, but with shipping, it’s almost always waaaay cheaper to pick up a bag at a home improvement or gardening store such as Lowes, Home Depot, gardening centers, etc. If possible, try to get a finer grade of dolomite compared to something that is more coarse.
How to Use Dolomite Lime for Cannabis: When growing cannabis indoors, add 6-7 teaspoons of fine dolomite lime to each gallon’s worth of soil. So if you’re mixing enough soil to fill a 5-gallon container, you want to add 30-35 teaspoons (about 2/3 cup) of dolomite lime to the mix. Mix the dolomite lime and the dry soil thoroughly, then lightly water it with water that has been pH’ed to 6.5. After getting the soil wet, mix the soil well and wait a day or two to let the soil settle before checking the pH and adding plants. When growing in an outdoor garden, follow the dolomite lime manufacturers instructions.
If watering plants by hand… Flush the system with properly pH’ed water that contains a full set of proper nutrients that are suitable for growing cannabis. Make sure you are using the right nutrients for the stage your plant is in. Check the pH of your runoff water to ensure that nothing in the growing medium is throwing off the root pH.
If growing hydroponically… Check the pH and PPM of your reservoir water to make sure that pH is on target and nutrient levels are not lower than expected. If you do this and are still not certain what is causing the magnesium deficiency, it is recommended that you drain your reservoir and refill with a newly mixed reservoir with fresh nutrients which have been pH’ed.
Should I add extra Magnesium? Some growers will add 1 tsp of Epsom salt/gallon of water and water plants with this mixture in response to a magnesium deficiency (since Epsom salt is primarily made of magnesium).
As I mentioned, often a magnesium deficiency is actually caused by a mix of factors, such as pH being off. Even if the pH is on target, sometimes a magnesium deficiency appears when other important nutrients like iron or calcium are not present in the right quantities.
A great supplement that has all of 3 of these important nutrients is known as Cal-Mag, as this contains Magnesium and Calcium, as well as a trace amount of iron.
Adding extra magnesium is often not necessary if you are using tap water. However, you will likely want to supplement Cal-Mag if you are using filtered or reverse osmosis (RO) water, since most tap water already contains some amount of all 3 of these cannabis nutrients. Cal-Mag also has a small amount or iron, which is another trace cannabis nutrient that is often missing in filtered water.
How long until new growth looks better? If you fix the root of the problem, further yellowing and discoloration of the leaves should stop almost immediately. Some of the affected leaves may recover somewhat, but what’s most important is to make sure the problem isn’t continuing to spread to other leaves on the plant.
I generally don’t remove any discolored leaves until I know for sure that the problem is completely gone and is no longer spreading to new leaves (that way any possible further discoloration will happen to the leaves that have already been affected).
- Bronze or brown patches
- Brown or slimy roots
- Brown or yellow leaf tips/edges
- Buds dying
- Buds look odd
- Bugs are visible
- Curling or clawing leaves
- Dark leaves
- Drooping plant
- Holes in leaves
- Mold or powder
- Pink or purple on leaves
- Red stems
- Shiny or smooth leaves
- Spots or markings
- Twisted growth
- Wilting leaves
- Yellow between leaf veins
- Yellow leaves
This page is part of our Plant Doctor series. You can use our tool to filter by symptom and help diagnose your plant.
Does your cannabis plant have yellowing lower leaves between the veins? See pictures of a magnesium deficiency and learn the easy solution.